1. Field of the Invention
The present invention relates to an optical waveguide coupler and its characteristic adjusting method.
2. Description of the Related Art
As shown in FIG. 3, an optical waveguide coupler (also called “directional coupler”) that includes an optical fiber coupler and is formed in optical waveguides inclusive of optical fibers, planar optical waveguides, and semiconductor thin film optical waveguide, is a device for exchanging optical power between two waveguides in a coupling portion 33 that is formed by placing part of two independent waveguides, a first waveguide 31 and a second waveguide 32, side by side in close proximity about five times the wavelength of transmitted light (see, pp. 45-47 of Hiroshi Nisihara, Masamitsu Haruna, and Toshiaki Suhara, “Optical Integrated Circuit (revised and enlarged edition)”, published by Ohmsha, Mar. 25, 1998).
Assume that the first waveguide 31 has a normal mode a, and the second waveguide 32 has a normal mode b, that the propagation constants of the two modes are βa and βb, respectively, where βb≧βa, and that the coupling coefficient of the two modes a and b is a coupling coefficient x in the coupling portion 33. When light with power 1 is incident onto the first waveguide 31, powers PI and PII of the outgoing light beams from the first waveguide 31 and second waveguide 32 are given by the following equations.PI=1−F sin2βcz  (1)PII=F sin2βcz  (2)where z is the length of the coupling portion 33, andβc=√{square root over (x2+δ2)},  (3)δ=(βb−βa)/2,  (4)F=1/[1+(δ/x)2]  (5)From these equations, the length of the coupling portion 33 that maximizes the PII, that is, that gives the maximum transfer of the light power from the first waveguide 31 to the second waveguide 32, is given by the following equation.z=π(2n−1)/2βc  (6)where n is an integer equal to or greater than one. The transfer rate of the power in this case is given by the foregoing F.
The coupling ratio, the most important characteristic of the optical waveguide coupler, is given by the ratio between the powers of the outgoing light beams from the two waveguides. As can be seen from the foregoing description, the coupling ratio is determined by the propagation constants βa and βb of the light beams in the two optical waveguides, the coupling coefficient x, and the length z of the coupling portion 33. For example, as for the coupler having the coupling portion 33 with the length z=π/2βc, the coupling ratio is given by 1−F:F. Where βa=βb, the value F is given by F=1. This means that the entire light incident onto the first waveguide 31 is emitted from the second waveguide 32 if the length of the coupling portion 33 is π/2βc.
As for fabrication methods of such a conventional optical fiber coupler, the following methods are known (see, Japanese Patent Application Laid-open Nos. 6-222239 (1994), 2000-338358, and 6-281837 (1994)). A first method heats part of two optical fibers that are placed in contact, and draws them along the direction of the length with application of heat, thereby fusing the two optical fibers with thinning them. A second method polishes a side of each optical fiber to get into D-shaped, and brings the polished surfaces of the two optical fibers into contact each other. Since these fabrication methods can make the coupler with monitoring its characteristics such as the coupling ratio, it can achieve desired coupler characteristics during the fabrication. Thus it is unnecessary for the fabrication methods to adjust the coupler characteristics after fabricating the coupler.
In contrast, as for the planar optical waveguide, it is fabricated as shown in FIGS. 4A-4E.
Preparing Si or SiO2 substrate 41 (FIG. 4A);
Depositing a lower cladding 42 thereon (FIG. 4B);
Depositing a core layer 43 thereon (FIG. 4C);
Patterning a coupling portion (33 of FIG. 3) at the same time when forming the core layer 43 into a desired optical circuit pattern 44 (FIG. 4D); and
Forming an upper cladding 45 by chemical vapor deposition method or flame hydrolysis deposition method (FIG. 4E).
As for the planar optical waveguide, the characteristic of the coupler can be measured only after completing the entire process because it cannot be monitored during forming the coupling portion. Accordingly, when any error occurs between the characteristics of the actually fabricated coupler and the desired characteristics, it becomes necessary to adjust the coupler characteristics after the fabrication.